A deactivating element of a generic type disclosed in DE 102 04 672 A1 is configured as a roller tappet. Coupling is achieved in this roller tappet by two pistons situated diametrically opposite each other in the inner element of the roller tappet. Each of these pistons comprises a flattened portion starting from its radially outer front end and extending on an underside of the piston. In the coupled state, these flattened portions cooperate with a counter annular surface of an annular groove provided in a bore of the housing of this prior art deactivating element.
As discussed in sufficient detail in said prior art document, two locking rings are provided for adjusting the coupling lash (besides this, a similar lash adjustment, but only with one ring, is disclosed in DE 101 46 131 A1). A constant-thickness lower locking ring and a variable-thickness upper locking ring are kept ready. The aforesaid annular groove must have a minimum height corresponding to a height of the piston in the coupled region (flattened portion), plus a coupling lash of, for example, 0.1 mm, plus the thickness of the thickest upper locking ring that may have to be mated thereto.
An annular space situated in front of the piston and defined by the annular groove serves as a pressure chamber for the hydraulic medium. Particularly in the case of deactivating elements with a very short overall length, a leak gap situated between the inner element and the housing and communicating with the aforesaid pressure chamber has only an inadequate height, so that excessive losses of hydraulic medium through this gap must be taken into account. Besides this, due to the relatively high annular space, an unnecessarily large quantity of hydraulic medium has to be accumulated for displacing the pistons. This has an unfavorable effect on the speed of activation and deactivation.